Electricity meters sometimes incorporate timing functions for billing metrics. For example, timing functionality is useful in “time-of-use” metering where the rate for energy can change depending on the time of day the energy is used. Meters employing time-of-use (“TOU”) metering employ a real time clock that is often based on a precision timing crystal. Such meters are typically required to maintain the real time clock through a power outage so that the billing schedule may resume in a proper state once power is restored.
The real time clock function can be maintained through a power outage using an auxiliary power source such as a battery. Power outages, however, can create issues with real-time clock accuracy. In particular, when utility power is available in the meter (i.e. the normal case), the real-time clock can self-calibrate or self-adjust based on the power line signal, which is typically an accurate, 60 Hz signal. During a power outage, however, the precision source is not available and drift of the free-running real time clock is possible. Moreover, in some international markets, the power line signal may not have a reliable and precise frequency.
There are industry standards for accuracy of the real time clock in utility meters. In particular, ANSI C12.1 2001 requirements for real time clock accuracy are two (2) minutes per week or 200 ppm over a temperature range of 30° C. to 70° C. Some utilities require greater accuracies such as 1 minute per month or 23 ppm at temperatures of 30° C. and 40° C. It is sometimes necessary to verify that the real-time clock is operating within the defined parameters to ensure compliance with the relevant standard.
Traditional methods of verifying clock accuracy are not always convenient as standards for clock accuracy become more restrictive. For example, prior methods tended to work fine when accuracy requirements were broad such as 200 ppm. However, verifying clock accuracy when limits are in the range of 23 ppm resulted in lengthy measurement times or complicated test setups that were not very practical in verifying accuracy on large numbers of meters. For example if a meter's clock can only be read to a resolution of 1 second and it is desired to determine accuracy to a resolution of 1 ppm, then the measurement time required would be 1,000,000 seconds or 11.57 days.
It is also desirable to verify clock accuracy without removing the meter cover or breaking the meter cover seal if one is used. Consequently, a nonintrusive method of quickly verifying timing accuracy is desirable. If a method of measuring clock accuracy is sufficiently brief, it may be practical to calibrate the meter's real-time clock during production or manufacturing. Currently, the energy measurement function of the meter can be calibrated during manufacturing.
There is a need, therefore, for a method and/or apparatus that provides a more time-efficient clock measurement. There is also a need for such a method that can be carried out without removing the meter cover.